1. Field of the Invention
The present invention relates to a motor control device, and particularly a motor control device which can drive a synchronous motor formed of a rotor provided with magnets with high efficiency and high reliability.
2. Description of the Background Art
In recent years, environmental issues have become an object of public concern, and great attention has been given to saving of energy. Particularly, in the field of electric motors, it has been desired to produce a motor having small sizes as well as high efficiency and high output power in view of saving of energy.
Motors such as a dielectric motor and an SPM (Surface Permanent Magnet) motor, which is provided with permanent magnets fixed to a surface of a rotor, are typical examples of the motors in the prior art, and these motors are superior in mass productivity.
Further, motors having structures different from the conventional structures have been developed. Among these motors, an attention has been given to an IPM (Interior Permanent Magnet) motor, in which permanent magnets for further increasing the efficiency are embedded in a rotor for utilizing a reluctance torque in addition to a Fleming torque.
FIG. 35 shows an example of a structure of the IPM motor. The IPM motor shown in FIG. 35 includes a rotor formed of a rotor core 131, which is formed of an iron core having a high magnetic permeability or layered ferrosilicon plates, and permanent magnets 132 embedded in rotor core 131. The IPM motor shown in FIG. 35 is a four-pole motor, in which four permanent magnets 132 are arranged such that N- and S-poles are arranged alternately to each other in the circumferential direction, although FIG. 35 shows only half a section.
In FIG. 35, a reference number 134 indicates a unit around which a coil is wound, a reference number 135 indicates a stator and a reference number 136 indicates teeth. According to this structure, a difference occurs between an inductance Ld in a direction of a d-axis extending from a center of permanent magnets 132 to a center of a rotor core 131 and an inductance Lq in a direction of a q-axis shifted by an electrical angle of 90 degrees from the d-axis. Thereby, a reluctance torque Tr occurs in addition to a Fleming torque Tm.
The relationship between them is analyzed in xe2x80x9cRotary Machine Employing Reluctance Torquexe2x80x9d (Nobuyuki Matsui, et al., T. IEE Japan, Vol. 114-D, No.9, 1994), which will be referred to as a xe2x80x9creference 1xe2x80x9d hereinafter. According to the reference 1, the relationship between Fleming torque TM and reluctance torque Tr satisfies the following formula (1).                                                         Tt              =                              Tm                +                Tr                                                                                        =                                                                    Pn                    ·                    φ                                    ⁢                                      xe2x80x83                                    ⁢                                      a                    ·                    ia                    ·                    cos                                    ⁢                                      xe2x80x83                                    ⁢                  β                                +                                                      Pn                    ·                                          1                      /                      2                                        ·                                          (                                              Ld                        -                        Lq                                            )                                        ·                                          ia                      2                                        ·                    sin                                    ⁢                                      xe2x80x83                                    ⁢                  2                  ⁢                  β                                                                                        (        1        )            
where Pn represents a number of pair of poles, xcfx86a represents a flux linkage, Ld indicates an inductance in the d-axis direction, Lq represents an inductance in the q-axis direction, id represents a current in the q-axis direction, xcex2 represents a current phase and ia represents a magnitude of a current vector.
As current phase xcex2 changes, Fleming torque Tm, reluctance torque Tr and a total torque Tt change as described below with reference to FIG. 36. As shown in FIG. 36, Fleming torque Tm takes on a maximum value when current phase xcex2 is 90 degrees, decreases as current phase xcex2 changes from 90 degrees, and becomes equal to 0 degrees when current phase xcex2 is 180 degrees. In contrast to this, reluctance torque Tr takes on a maximum value when current phase xcex2 is 135 degrees. Therefore, total torque Tt which is a sum of reluctance torque Tr and Fleming torque Tm takes on a maximum value when current phase xcex2 is equal or close to 115 degrees although it depends on a torque ratio. Accordingly, the IPM motor which effectively utilizes reluctance torque Tr can issue a higher torque than the SPM motor operating only with Fleming torque Tm, if these motors use the same current.
A motor drive controlling method is a major factor for determining a magnitude of the torque of the motor. In a conventional current drive method, 120xc2x0 rectangular wave drive is generally performed. According to this 120xc2x0 rectangular wave drive method, a current is supplied to two among three (U, V and W) phases of motor coils so that the currents joined at every 120 degrees form a direct current, and thereby an inverter is controlled. According to the 120xc2x0 rectangular wave drive, an unconduction period is provided for every phase, and an induced voltage which is generated in the stator coil by rotation of a rotor magnet during this unconduction period is detected for controlling the rotor rotation. In the IPM motor utilizing reluctance torque Tr described above, the conduction timing is important conditions that can maximize the torque. In the IPM motor, therefore, the 120xc2x0 rectangular wave drive is performed, and the induced voltage is detected during the unconduction period for calculating the rotor phase.
In contrast to this, a 180xc2x0 sinusoidal drive method in which the conduction width is set to 180 degrees in electrical angle may also be employed as a motor drive control method for improving the motor efficiency. According to xe2x80x9cMethod of Controlling Driving of Brushless DC Motor, and Apparatus Therefor, and Electric Machinery and Apparatus Used Thereforxe2x80x9d (International Laying-Open No. WO95-27328), which will be referred to as a xe2x80x9creference 2xe2x80x9d, the conduction width is set to 180 degrees in electrical angle in a motor provided with embedded permanent magnets, and positions of magnetic poles are detected based on differences between a first center point potential of the motor coil and a second central point potential attained by a bridge circuit which is electrically parallel to the motor coil.
A brushless DC motor control device disclosed in the reference 2 will now be described with reference to FIG. 37. FIG. 37 schematically shows a structure of a motor control device disclosed in the reference 2. In FIG. 37, an inverter is formed by employing three switching transistor pairs 212u, 212v and 212w, each of which is connected in series between terminals of a DC power supply 211, and the voltage on the connection line between the switching transistors in each pair is applied to corresponding one of Y-connected stator windings 213u, 213v and 213w of the respective phases in the brushless DC motor. The voltage on the connection point between the switching transistors in each pair is also applied to corresponding one of Y-connected resistances 214u, 214v and 214w. A voltage on a neutral point 213d is applied to an inverted input terminal of an amplifier 215 via a resistance 215a, and a voltage on a neutral point 214d of the Y-connected resistances is applied to a noninverted input terminal of amplifier 215. By connecting a resistance 215b between an output terminal and the inverted input terminal of amplifier 215, the structure can operate as a differential amplifier. A voltage En0 on neutral point 213d among stator windings 213u, 213v and 213w is equal to a sum of an inverter output waveform and a 3n-th (n: integer) harmonic components contained in the motor induced voltage waveform. A voltage on neutral point 214d among Y-connected resistances 214u, 214v and 214w is determined only by the output waveform of the inverter. Therefore, the 3n-th harmonic components contained in the motor induced voltage waveform can be taken out by obtaining the difference between voltage En0 on neutral point 213d and the voltage on neutral point 214d. By the foregoing manners, the motor induced voltage waveform, i.e., the rotor position can be detected without using the magnetic pole position sensor, and therefore the 180xc2x0-drive method can be achieved.
xe2x80x9cController for Electric Vehiclexe2x80x9d (Japanese Patent Laying-Open No. 10-341594, which will be referred to as a xe2x80x9creference 3xe2x80x9d hereinafter) has disclosed a structure, in which the 120xc2x0 drive method or the 180xc2x0 drive method are selected, if necessary, when an abnormal condition occurs in a magnetic pole position detector or a rotary pulse detector.
According to the structure of the reference 2 described before, an external circuit such as a differential amplifier is provided for resistance connection 214u, 214v and 214w providing the center point of the motor coil connection so that the rotor position can be detected in the 180xc2x0 sinusoidal conduction state.
In the synchronous motor, the 120xc2x0 rectangular wave drive method may provide higher efficiency than the 180xc2x0 sinusoidal drive method in some cases depending on the state (e.g., output and rotation speed) of the motor, and therefore driving only by the 180xc2x0 sinusoidal drive method cannot always provide the optimum efficiency.
The system in the reference 3 is aimed at dealing with, e.g., a situation in which abnormal conditions occur in a rotary pulse detecting circuit in the control device for the electric vehicle not provided with the magnetic pole position detecting circuit, or a situation in which abnormal conditions occur in both the magnetic pole position detector and the rotary pulse detecting circuit in the control device for the electric vehicle provided with the magnetic pole position detector. Therefore, the system of the reference 3 cannot be the optimum system in view of efficiency.
The 120xc2x0 drive method is executed in the case where an abnormal condition occurs for the purpose of continuing the driving without stopping the motor, and the control method during this driving is based on an estimated magnetic pole position which is estimated by a magnetic pole position estimating circuit. Accordingly, disadvantages relating to efficiency cannot be overcome at all.
In the prior art, a sensor-less drive method for controlling and driving a synchronous motor without using a motor rotor position sensor employs the following intermittent-conduction drive. According to the intermittent-conduction drive, a predetermined unconduction period is present in the operation of conduction the motor coil, and a counter electromotive voltage, which is generated in the motor coil by rotation of the motor during the unconduction period, is detected through a motor coil terminal, so that the conduction timing is determined in accordance with this counter electromotive voltage. According to this conduction drive method, a so-called 120xc2x0-conduction drive method such as 120xc2x0 rectangular wave drive is generally employed.
Alternatively, so-called 180xc2x0-conduction drive such as sinusoidal conduction drive may also be employed, in which case the synchronous motor is driven without providing an unconduction period. More specifically, such a method may be employed that resistances are connected in parallel to a neutral point of three-phase motor coils and the three-phase motor coils, and the voltage on the neutral point is compared with the voltage on the resistance neutral point for detecting the motor electromotive voltage for determining the conduction timing of the motor and thereby driving the motor. Also, such methods may be employed that fast arithmetic of the motor current is performed for detecting the motor position, and thereby the conduction timing is determined for driving the motor, or that the motor is driven by determining the conduction timing based on the phase difference between the motor drive voltage and the motor current.
Generally, the 180xc2x0-conduction drive method provides a smoother drive waveform than the 120xc2x0-conduction drive, and therefore causes less variations in torque and rotation speed.
In the synchronous motor of a permanent magnet rotor structure, conduction of the motor is performed in accordance with accurate timing corresponding to the position of the permanent magnet, and the optimization of this conduction timing is essential for driving the motor. In addition to this essential condition, the conduction timing must be set to the optimum timing depending on the respective rotation conditions for achieving high efficiency and stable rotation.
According to the intermittent-conduction drive such as 120xc2x0-conduction drive, a counter electromotive voltage related to a permanent magnet flux and an armature flux is directly detected, and the permanent magnet position and thus the rotational position are actually detected. Therefore, the motor drive can be performed in accordance with accurate conduction timing by improving the detection accuracy, e.g., by removing noises. More specifically, since the motor rotational position is directly detected, disadvantages such as stop of the motor can be suppressed even when a disturbance is applied.
Compared with the intermittent-conduction drive such as 120xc2x0-conduction drive, the 180xc2x0-conduction drive without a position sensor can improve the efficiency and can reduce noises and vibrations more effectively, but the 180xc2x0-conduction drive without a position sensor generally suffers from complicated drive and control. This is due to the fact that the motor rotational position is not directly detected, and the detection of the conduction timing is performed with a low accuracy. Therefore, the disadvantage such as stop of the motor is likely to occur when a disturbance is applied.
For example, according to the 180xc2x0-conduction drive method in which the conduction timing is determined based on a comparison between the coil neutral point and the resistance neutral point, the conduction timing of the drive voltage is controlled. However, it is the motor current that actually determines the motor torque. According to the 180xc2x0-conduction drive not employing an off period, a phase difference occurs between the drive voltage and the motor current due to an influence by the counter electromotive voltage of the permanent magnet and the coil inductance. If this difference is deemed as the conduction timing, the sensitivity to the motor current is higher than the drive voltage. From experiments, such a result was obtained that the sensitivity increases two through three times compared with the intermittent-conduction drive under some rotation conditions. Therefore, very strict detection of the conduction timing is required. Thus, the 180xc2x0-conduction drive requires the accuracy which is higher by two to three times higher than that in the intermittent-conduction drive.
According to the 180xc2x0-conduction drive method in which the motor current is analyzed by fast arithmetic for determining the conduction timing, the detection resolution of the conduction timing is usually impaired by an electrical angle of about 5 degrees compared with the intermittent-conduction drive due to a detection error, an arithmetic error, an arithmetic delay and others of the motor current.
Further, according to the 180xc2x0-conduction drive method based on the phase difference between the motor drive voltage and the motor current, the conduction operation is switched in accordance with elapsing of time by so-called forced excitation, and the motor current phase difference at the time of this switching and thus the conduction timing are controlled. However, the error in control of the motor current phase difference directly results in the error in conduction timing. Therefore, it is necessary to control strictly the phase difference for achieving the stable driving and maintaining the motor rotation. This restrict control can be performed when no disturbance occurs, but the control becomes particularly instable when the disturbance occurs. The conduction timing in the intermittent-conduction drive depends on the detected counter electromotive voltage, and therefore the accurate conduction timing can be achieved regardless of the control performance. Accordingly, the phase difference control requires more accurate and strict control than the intermittent-conduction drive.
As described above, the 180xc2x0-conduction drive requires the accurate and strict control. Therefore, it cannot achieve efficient drive if disturbances reducing the control margin occurs. Further, it suffers from problems such as error in motor conduction control and stop of the motor. The possibility of occurrence of these problems is extremely higher than that in the intermittent-conduction drive such as 120xc2x0-conduction drive.
The above disturbances specifically include changes in power supply voltage supplied to the inverter driving the device or synchronous motor, changes in motor rotation speed and changes in load torque. In the 180xc2x0-conduction drive, the control is generally difficult as compared with the intermittent-conduction drive such as 120xc2x0-conduction drive, and therefore the robustness against the disturbance is generally low.
As described above, the 180xc2x0-conduction drive method is superior in efficiency, torque vibrations, rotation vibrations and noises, but has low control robustness. Further, according to the 180xc2x0-conduction drive method, the control performance itself is improved, e.g., by raising the control gain as measures against the disturbances. However, the disturbances which cannot be covered by the improved control performance may cause problems such as stop of the motor because measures cannot be taken against such disturbance.
As already stated, the reference 3 has disclosed the structure for switching the operation between the 120xc2x0-conduction drive method and the 180xc2x0-conduction drive method.
In the structure disclosed in the reference 3, however, a rotational pulse generating circuit such as an encoder is used during the 180xc2x0-conduction drive, and a position sensor for detecting the motor position is required. Accordingly, the structure in the reference 3 cannot be utilized in the structure for driving the motor without a position sensor.
According to the system of the reference 3, the drive method is switched in such a manner that the 120xc2x0-conduction drive is selected when an output of neither a position sensor nor a rotation pulse generating circuit can be obtained, or that the 180xc2x0-conduction drive is selected in a low speed range where a counter electromotive voltage cannot be detected without difficulty, and the 120xc2x0-conduction drive is selected in the middle and high speed range. Thus, the selection is performed only based on the rotation speed. Accordingly, it is impossible to deal with the specific conditions of occurrence of disturbances affecting the motor driving, and therefore it is impossible to achieve the motor driving with high efficiency, low noises, low vibrations and high reliability.
An object of the invention is to provide a motor control device which can efficiently drive a synchronous motor including a rotor unit provided with a magnet.
Another object of the invention is to provide a motor control device which can drive a synchronous motor including a rotor provided with a magnet with high efficiency and high reliability without using a position sensor while dealing with disturbances.
In summary, the invention provides a motor control device including a drive control circuit for controlling a synchronous motor. The drive control circuit controls the drive of the synchronous motor. The drive control circuit includes a plurality of conduction drive circuits for conducting and driving the synchronous motor. The plurality of conduction drive circuits include at least a 180xc2x0-conduction drive circuit for performing 180xc2x0-conduction drive of the synchronous motor, and a 120xc2x0-conduction drive circuit for performing 120xc2x0-conduction drive of the synchronous motor. The drive control circuit selects one of the plurality of conduction drive circuits in accordance with a motor efficiency of the synchronous motor.
Accordingly, a major advantage of the invention is that optimum driving in view of the motor efficiency, and more specifically driving of the synchronous motor with the optimum efficiency can be performed by selecting the plurality of conduction drive circuits.
According to another aspect of the invention, a motor control device for controlling a synchronous motor includes a drive control circuit. The drive control circuit controls chive of the synchronous motor. The drive control circuit includes a conduction width corresponding drive circuit for arbitrarily setting a conduction width of the synchronous motor. The drive control circuit controls the conduction width corresponding drive circuit in accordance with a motor efficiency of the synchronous motor.
Accordingly, the synchronous motor can be driven with the optimum conduction width in view of the motor efficiency, and thus with the optimum efficiency.
According to still another aspect of the invention, a motor control device for driving and controlling a synchronous motor formed of a rotor provided with a permanent magnet and a drive circuit for driving the synchronous motor without using a position sensor, includes a 180xc2x0-conduction drive circuit, an intermittent drive circuit, a motor disturbance monitoring circuit and a drive method selecting circuit. The 180xc2x0-conduction drive circuit is provided for performing 180xc2x0-conduction drive of the synchronous motor. The intermittent-conduction drive circuit is provided for performing intermittent-conduction drive of the synchronous motor with an unconduction period and a conduction angle smaller than 180 degrees. The motor disturbance monitoring circuit is provided for monitoring a disturbance against the synchronous motor and the drive circuit. The drive method selecting circuit selects one of the 180xc2x0-conduction drive and the intermittent-conduction drive as a drive method of the synchronous motor in accordance with an output of the motor disturbance monitoring circuit.
Accordingly, the 180xc2x0-conduction drive circuit and the intermittent-conduction drive circuit can be appropriately selected in accordance with the disturbance so that the motor drive with high efficiency, low noises and low vibrations can be achieved during the steady state in which disturbances are small, and the motor drive with high reliability can be achieved without causes disadvantages such as stop of the motor during the unusual state in which the disturbances are detected.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.